The present invention relates to a wavelength selective optical recording and reproducing method.
An example of a conventional wavelength selective recording and reproducing device is shown in U.S. Pat. No. 4,101,976. FIG. 10 of this application shows the construction thereof schematically and FIGS. 11a, 11b and 11c correspond to FIGS. 3, 4 and 5 of the U.S. Patent, respectively.
In FIG. 10, the recording and reproducing device is constituted with a wavelength variable light source 10, a wavelength controller 11 for changing the wavelength of the light source 10, a collimator lens 12 for collimating light from the light source 10 into parallel beam, an optical deflector 13, an objective lens 14 for condensing light from the light source 10 to a minute spot and directing it onto a recording medium 15, a memory element 16 selected by the optical deflector 13 from memory elements of the recording medium 15 and an optical sensor 17 for detecting light passed through the selected memory element 16.
Light from the light source 10 is collimated by the collimator lens 12 to parallel light and after condensed by the objective lens 14 to a light spot directed onto the selected memory element 16 on the recording medium 15. The selection of the memory element is performed arbitrarily by the optical deflector 13. The principle of recording and reproducing of wavelength selective information on the selected memory element 16 will be described with reference to FIGS. 11a, 11b and 11c.
In FIG. 11a which shows an absorption spectrum of the recording medium prior to the wavelength selective recording, the spectrum is broadly spreaded. When such medium is irradiated with light having a light intensity spectrum such as shown by a dotted line in FIG. 11a, the absorption spectrum provides sharp negative spikes at wavelengths of the irradiating light as shown in FIG. 11b. The negative spike is called as "spectral hole". When such hole occurs, it is considered that a data "1" is stored at the corresponding wavelength and a data "0" is stored at any other wavelength at which no spectral hole occurs. In order to form a hole at any arbitrary wavelength, the wavelength of the light source 10 is regulated by the wavelength controller 11 to a value at the wavelength of the hole to be recorded and to increase the light intensity of the light source 10 to a value necessary to perform a recording. On the other hand, in order to readout signals from the medium on which information is recorded in wavelength selective mode such as shown in FIG. 11b, a wavelength scanning is performed from an upper limit A to a lower limit B of the memory wavelength range with the intensity of the light source 10 being constant. Since the absorptivity is reduced at the respective hole wavelength as shown in FIG. 11b, an optical intensity spectrum such as shown in FIG. 11c is obtained by detecting light that has passed through the medium 15 by using the optical detector 17. Although FIG. 11c shows the wavelength spectrum, it is possible to obtain a reproduced signal output at an output of the optical detector 17 by scanning the wavelength at a constant speed in time. The reproduced signal is in time-series, that is, a time series of signals forming information.
In the conventional wavelength selective optical recording and reproducing device constructed as above, a time series signal existing in a memory element which is wavelength-scanned to reproduce information contained therein will be lost completely if the memory element becomes inoperative for some reason.